1. Field of the Invention
This invention relates to an alternator for a vehicle such as a passenger automotive vehicle or a truck.
2. Description of the Related Art
To reduce the aerodynamic resistance in a traveling condition, a vehicle body tends to be formed into a slant nose shape. Securing a sufficient residential space for a passenger compartment is earnestly demanded. To satisfy these requirements, engine rooms of automotive vehicles have recently been becoming so narrow and crowded that only a limited space is available for installing an alternator. In addition, the temperature of a region around the alternator has been high. Meanwhile, to improve fuel economy, the rotational engine speed tends to be reduced during an idling condition. The rotational speed of the alternator decreases in accordance with the reduction of the rotational engine speed. On the other hand, there is a need for increasing electric loads on safety control devices or others. Thus, the power generating ability of the alternator is strongly required. In other words, a compact high-power alternator for a vehicle is required. Also, an inexpensive alternator for a vehicle is desired.
In a general alternator (a prior-art alternator) for a vehicle, as shown in FIG. 2, a rotor contains a Lundel-type iron core (referred to as the pole core hereinafter) having a cylindrical portion, a yoke portion, and a claw-like magnetic pole portion. The entire length of the general alternator is determined by the axial-direction length (referred to as the axial length hereinafter) of the rotor. Accordingly, a reduction in the axial length of the rotor is desired for a compact alternator design.
In the rotor of the general alternator, as shown in FIG. 2, magnetic flux .PHI. flows from the cylindrical portion to the yoke portion and the claw-like magnetic pole portion, gradually advancing from the claw-like magnetic pole portion to a stator iron core. The magnetic flux .PHI. generated from the rotor is given as follows. EQU .PHI.=AT/G
where "A" denotes a current flowing in a field coil; "T" denotes the number of turns of the field coil; and "G" denotes the sum of the magnetic resistances of respective portions. The term "AT" indicates the product of the current "A" and the turn number "T" which is proportional to the square root of the cross-sectional area of the field coil. The product "AT" is also referred to as the coil "AT". Each of the magnetic resistance is proportional to the length of a magnetic path which is divided by the cross-sectional area of the magnetic path.
In the prior-art structure of FIG. 2, the magnetic-path cross-sectional areas S1, S2, and S3 at different portions of the pole core are set substantially equal to each other to prevent the occurrence of local magnetic saturation. The dimensions of the portions of the pole core are chosen to provide a proper space for the field coil. The cross-sectional area of a magnetic path in the stator iron core is made substantially uniform in correspondence with the magnetic flux generated by the rotor. The cross-sectional area of each slot in the stator iron core is decided on the basis of the resistance of a winding. As a result, the axial length of the stator is also decided.
In a prior-art magnetic circuit which is designed in such a way, the axial length L2 of the cylindrical portion of the pole core is substantially or approximately equal to the axial length L1 of the stator iron core as shown in FIG. 2.
In the prior-art structure of FIG. 2, when an increased alternator power output is required, the magnetic flux generated by the rotor is increased. To implement the generation of an increased magnetic flux, it is necessary to increase the coil "AT" (that is, enlarge the field coil) or to reduce the magnetic resistance.
In the prior-art structure of FIG. 2, if the generated magnetic flux is increased without changing the size of the rotor, the cross-sectional areas of the respective portions of the rotor need to be reduced to allow an increase in the cross-sectional area of the field coil to enhance the coil "AT". If a greater cross-sectional area of the magnetic path is required to reduce the magnetic resistance, it is necessary to reduce the cross-sectional area of the field coil. Thus, the prior-art structure of FIG. 2 needs to consider a trade-off between the two requirements.
FIG. 3 shows another prior-art structure in which the dimensions of a rotor are equal to those of the rotor in the prior-art structure of FIG. 2 and the axial length L1 of a stator iron core is set longer than the axial length L2 of a cylindrical portion of a pole core to reduce the magnetic resistance of the stator iron core and the air gap between the rotor and the stator. If the magnetic resistance of the stator side is reduced to increase magnetic flux, only a small increase in the magnetic flux is available since the amount of the magnetic flux is limited by magnetic saturations in the respective portions of the rotor. In this case, there hardly occurs an improvement of an alternator power output per unit weight since an increase in the weight of the stator serves as a cancelling factor.
FIG. 4 shows still another prior-art structure in which the cross-sectional area S3 of a base of a claw-like magnetic pole portion is set smaller than the cross-sectional area S1 of a cylindrical portion and the cross-sectional area S2 of a yoke portion to allow a great cross-sectional area of a field coil. In the prior-art structure of FIG. 4, the base of the claw-like magnetic pole portion tends to magnetically saturate. Therefore, although the cylindrical portion and the yoke portion have sufficient magnetic capacities, the magnetic resistance of the base of the claw-like magnetic pole portion tends to abruptly rise so that the amount of magnetic flux is significantly limited.
In the prior-art structure of FIG. 4, the magnetic flux is blocked by the claw-like magnetic pole portion so that the magnetic flux leaks along an axial direction. The leak of the magnetic flux results in a reduction of magnetic flux reaching a stator iron core. The reduced magnetic flux in the stator iron core decreases an alternator power output.
In a known field coil occupying a narrower space, the resistance of the coil is reduced and a field current is increased to provide a proper coil "AT". In this case, the rate of heat generated by the field coil increases so that a problem occurs in the cooling thereof. Also, there occur the problems that the excitation loss rises and the power generation efficiency drops.
Japanese published unexamined patent application 61-85045 discloses that a magnet is inserted into a region between claw-like magnetic pole portions to prevent a leak of magnetic flux and thereby to increase the amount of magnetic flux in a rotor. In this case, a cost problem occurs due to the magnet. In addition, it is necessary to provide a device for retaining the magnet. The retaining device increases the cost. Furthermore, there is a chance that the magnet moves out of the normal position due to a centrifugal force. Therefore, it is difficult to practically use the prior-art structure of Japanese application 61-85045.